This invention relates to a white light emitting luminaire having an array of red, green and blue light emitting diodes (LEDs) and a control system for adjusting the light output of the individual components to maintain a desired color balance (chromaticity), and more particularly relates to such a luminaire having specific light sensor configurations for optical feedback.
White light emitting luminaries having arrays of red, green and blue LEDs (also known as RGB LED luminaries) are of interest for several reasons, including efficiency and low cost, and the ability to adjust the chromaticity of the light output.
One of the central problems to be addressed is the variation in light output of the LEDs from chip to chip, as well as over the life of each chip. Light output of the LEDs also varies inversely with temperature, but not uniformly for each color. In addition, light output will vary with the failure of individual chips within an LED array.
U.S. Pat. No. 6,127,783, issued to Michael D. Pashley and Thomas M. Marshall and assigned to Philips Electronics North America Corp., discloses a white light emitting luminaire with electronically adjusted color balance. The luminaire includes a plurality of LEDs in each of the colors red, green and blue with a separate power supply for each color and a photodiode arranged to measure the light output of all the LEDs. The light output of each color is measured by an electronic control circuit which turns off the LEDs for the colors not being measured in a sequence of time pulses. The measured light output for each color is compared to a desired output, which may be determined by user inputs, and corrections to the current for each color are made accordingly. The specification of U.S. Pat. No. 6,127,783 is hereby incorporated by reference into this specification.
In order to accurately control the output of such a luminaire, the total delivered light must be monitored accurately. This requires placing the photodiodes in such a manner that an equal fraction of light is sampled from each LED while allowing sufficient stray light from the LEDs to fall on the photodiode(s) to insure satisfactory operation of the feedback loop.
The invention provides several optical configurations for positioning one or more photodiodes, optionally with various color filters, in and around the lightpath of a RGB LED luminaire to achieve an equal fraction of light sampled from each LED in order that the total light output can be monitored accurately. Only minor modifications to a standard RGB LED luminaire""s optical system are needed, and efficiency and other performance specifications are substantially unaffected. With the photodiodes so positioned, feedback signals are available to the control electronics to regulate both total light output and color balance.
In a first embodiment of the invention, a separate photodiode is incorporated into each LED package, which includes an LED chip and a lens to direct the light from the LED. The photodiode may be placed at a location to receive light directly or indirectly from the LED. Simulations have shown that reflections off the inner surface of the lens will allow sufficient stray light to fall on the photodiode to insure satisfactory operation.
In a second embodiment, a separate photodiode package is positioned immediately adjacent to each LED or a small group of LEDs (eg., 3 LEDs), with its entrance aperture or collection lens directed towards the LED to be monitored.
In a third embodiment, a partially reflecting element (typically about 1% reflection is sufficient) is positioned within the lightpath, after the main condenser lens, the partially reflecting element positioned and shaped to reflect a small portion of the output beam back through the condenser lens to focus at or slightly above the intersection of the optic axis with the plane of the LED array. At this focus is placed a light sensor assembly, including one or more photodiode(s), a white integrating chamber surrounding the photodiodes and a diffuser such as a planar diffusing screen between the chamber and the condensing lens. The light sensor assembly is arranged such that over a significant area (several times the size of one photodiode package) the flux from each LED is nearly constant. While this embodiment is the most elaborate, it offers the best performance, economy, and versatility.
In each of the embodiments, color filters may be associated with the photodiodes to render them selective to a particular spectral region of the RGB output, thus avoiding the need to pulse the LEDs and photodiodes as described in U.S. Pat. No. 6,127,783.